1	CS110 Lecture 13 Thursday, March 11, 2004 Announcements hw5 due tonight Spring break next week hw6 due two weeks from tonight Agenda questions TreeMap hw6 stub programming
2	Maps arrays and ArrayLists locate entries by index index is an integer position, starting at 0 A Map locates entries by key key is often a String (think dictionary, phone book) A map stores key-value pairs key: “Java” value: “a modern OO language” key: “UMass” value:
3	TreeMapDemo.java Class Integer to wrap primitive int Iterator practice Useful toString (for debugging) Remember to cast Same value stored twice Keys that aren’t Strings
4	Getting an Iterator Set keys = map.keySet(); Iterator keysIterator = keys.iterator(); ask the map for its keySet ask the keySet to give you an Iterator keysIterator is like a list of the keys in the map You can infer from this code that Set and Iterator are classes in the Java API keySet is a method in class TreeMap; it returns a Set iterator is a method in class Set; it returns an Iterator
5	Using an Iterator while ( keysIterator.hasNext() ) { String key = (String)keysIterator.next(); terminal.println( " … " + (Integer)map.get( key)) ); } hasNext() returns false when at end of list next() returns a reference to the next Object in the list Iterator next method, like TreeMap get method, returns just an Object. Need two casts.
6	Class Lookup Dictionary has no unit test Class Lookup is a client for Dictionary (and tests it thoroughly) All of Lookup is static > java Lookup <word> <word> … all Lookup.java sends toString messages to a Definition object (line 53) and to a Dictionary object (line 103)
7	Dictionary boxes and arrows
8	Looping on a TreeMap To print the whole Dictionary, Lookup sends a toString message, invoking Dictionary toString method (line 70) Subtle, since there’s no index to loop with Uses an Iterator object - Java tool custom designed for looping Iterator API has just two methods: boolean hasNext() Object next()
9	Building a multiline String while ( wordIterator.hasNext() ) { word = (String)wordIterator.next(); definition = this.getEntry( word ); str += word + ":\n" + definition.toString() + "\n"; } use the key to look up a Definition send the Definition a toString message add two lines to the String str we are building to represent the whole Dictionary
10	TreeMap summary declaration: TreeMap mapName; creation: new TreeMap( ); put: mapName.put(Object key, Object obj) get: (Type)mapName.get(Object key) cast to proper Type length: mapName.size( ) looping: get Set of keys from the map, then get an Iterator from the set mapName.keySet( ).iterator( )
11	Duplicates? The same value may appear more than once in a collection (array, ArrayList or Map) my wife and I have the same phone number “field” and “instance variable” have the same definition in an array, foo[3] may == foo[7] In a Map, keys are unique (like index in an array) If you want to arrange for one person to have more than one phone number or one word to have more than one definition you need to work harder …
12	Collections of collections Dictionary might map a word to an ArrayList of Definition instances Registrar’s database maps a student ID to a StudentRecord object that contains a Map of courses taken (key course name, value an object storing the grade) Screen maintains a private field that’s an array of arrays of char: private char[][] pixels;
13	Figure 4.5 Object structure of a 2x3 Screen
14	hw6 Due Thursday after Spring break TreeMap practice class Directory little Bank, using a TreeMap
15	Directory Model windows folder Contains TextFiles (not other Folders - wait for Chapter 5) API: create, add file to, get file from, get size, get owner, get create/mod date Design: Directory object has a TreeMap field storing TextFile objects keyed by String filename You write this for homework
16	Stub programming Start with all javadoc comments and method declarations, empty method bodies Fill in code a method at a time (perhaps even a line at a time) Compile and test as you go along!